E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
To express certain political views. The allusion is to the custom, in Queen Annes reign, of wearing on the face little black patches. If the patch was on the right cheek, it indicated that the wearer was a Whig; if on the left cheek, that she was a Tory; if on the forehead between the eyes, or on both cheeks, that she was of no political bias. (See COURT PLASTER.)
Whatever might be her husbands politics, she was at liberty to patch as she pleased.Nineteenth Century, February, 1890, p. 58.